Blood flow keeps tissue alive. Accordingly, its measurement and monitoring present a significant medical need. Attempts have been made to develop blood flow measurement in vessels for more than 20 years. Particularly advantageous would be means of measuring flow in vessels inside the body, in either natural blood vessels or artificial blood vessels known in the art as prosthetic grafts.
Grafts, commonly replacing diseased natural blood vessels for example, often fail in a relatively short time. For example, when placed in the leg (60,000 grafts a year) data supports that only about one-third of implants still function after five years, and that one-third of the legs have nonetheless been amputated. More particularly, when first placed, graft flow is often high. With time, however, stenoses in the connecting vessels reduce flow such that blood clots in the graft and the graft “fails”. Once a graft fails, the prognosis is grim for a limb sustained by the graft—such that amputation is often necessary.
While methods have been proposed to mount flow-sensors in the body and to notify the outside world of diminished flow by means of such implanted systems, it is believed this long felt need has not yet been met.
Doppler ultrasound is useful for assessing flow. To measure the velocity by Doppler effect, however, the measuring ultrasound beam must have a substantial component of its direction in the direction of flow. More particularly, the Doppler shift frequency, Fdop may be found from:
                              fdop          =                                    (                              v                λ                            )                        ⁢                                                  ⁢                          cos              ⁡                              (                θ                )                                                    ,                            (        a        )            where v is the velocity of the back-scattering material, λ is the wavelength of the ultrasound used, and θ is the angle between the insonifying beam and the velocity vector.
However, when a Doppler transducer is conventionally placed flat against the wall of the vessel, the angle between the insonifying beam from the transducer and the velocity is 90°, such that cos(θ) is zero, yielding no flow information. This has prevented using an embedded Doppler transducer in the wall of a graft: as even if the transducer is only 1 mm in length, when angled at just 30° it cannot fit inside the 0.5 mm wall of a graft.